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99 STS
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Discussion Starter #1
I have a 99 Seville STS with 149,000 miles. No history codes and no current codes. I'm getting 15-20 misfires on #8 cylinder every startup. I switched #8 spark plug, wire and coil with #6 plug, wire and coil. Still misfires on #8 cylinder. Switched them back. Eliminated those 3 things. Switched ignition module with friend's 2000 STS. Still misfires #8 cylinder. Eliminated that. Installed new fuel injector on #8 cylinder. Still misfires on #8. Performed compression test on #2, #4, #6 and #8 cylinders. All were in the same range. I've ran out of ideas! Any suggestions would be appreciated.
 

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White Diamond '03 DHS (with DTS floor shift)
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Re: 1999 Seville STS-Misfire

Are you overheating or using coolant at all?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Swapped ICMs ??? A 2000 or later ICM bears no resemblance to a 1999 or earlier. Entirely different ignition systems. Entirely. *Unless you are outside the U.S. and the "2000" actually has a build date of between 07/98 and 06/99. THEN it's a 1999 car.

Here's a 2000 and later coil cassette with ICM. Your 1999 should have 4 coils mounted on the right (rear) cam cover.

Have you checked the ICM ground wire - and are the coils properly connected to their respective spark plugs ?

EDIT: *Texas was in the U.S. last time I looked ................
 

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99 STS
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93 Posts
Discussion Starter #5
My friend has salvage yard. He said it came off of a 2000. He was definitely wrong because the one he brought me was identical to mine. In fact, I left it on mine.

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I don't see a ground wire on ICM. Just 2 wire harnesses on each end. Doesn't it ground itself on metal base it's bolted to? The coils are properly connected to their respective spark plugs.
 

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White Diamond '03 DHS (with DTS floor shift)
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The ground on your ICM is not a wire. It's grounded by mounting. Did you install the hidden bolt in back? That might be the ground.
 

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98 DeVille, 97 DeVille d'Elegance
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The ICM grounds to the aluminum plate it is mounted to however that aluminum plate has a ground wire runs behind the engine. It is commonly left off as its a bitch to see. So technically the ICM does have a ground wire. It can cause some issues if left off, it's a heavy gauge wire with a ring terminal on the ends.
 

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Discussion Starter #10
Back bolt was missing from ICM. Replaced it. There wasn't a ground wire on ICM. I made one with ring terminal on each end and installed it. 0 misfires on every cylinder now. Thanks guys!
 

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White Diamond '03 DHS (with DTS floor shift)
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I think that back bolt alone would have done the trick. I was sure there was no ground wire, but adding one could not hurt.
The ground icon in Sub's diagram does not indicate a wire. Just that the unit is grounded.
 

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98 DeVille, 97 DeVille d'Elegance
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The Cam cover doesn't provide a ground. My 97 has the ground on the opposite side right where you can see if and not forget it. I left it off my 98 once during plug change and it ran weird, it ran just not quite right.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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This ground wire and the problems associated with it has been discussed in the Engines and Tech Tips forums for years. The OEM wire - black with ring terminals - went from the ICM base over to a bolt near the P/S pump.

EDIT: I've disconnected a few when prowling the junkyards with the car club. The pre-2000 waste spark system is a good swap-in for some older engines and it adapts well to external crank trigger wheels. One reason why the systems, as well as coils, are becoming a bit scarce in junkyards.
 

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White Diamond '03 DHS (with DTS floor shift)
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I pulled the ICM on my '97 when I had the misfire from the fuel rail recall and there was no ground wire that I recall. Key word is recall, but I'd swear there is none.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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I had forgotten to reconnect the said ground wire from the ICM during the engine overhaul - found it before starting the car for the first time, but I forgot where it connected originally, so I connected to any ground it could reach.
 

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White Diamond '03 DHS (with DTS floor shift)
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:hmm: Well I guess my memory has failed me and I stand corrected once again. That ground wire pic does seem to trigger the slightest of current flow in my memory bank.
 
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